pocket book vs softback book

pocket book and softback book both are nouns.

Word	Noun	Adjective	Verb	Adverb
pocket book	Yes	No	No	No
softback book	Yes	No	No	No

As nouns, softback book is a hypernym of pocket book; that is, softback book is a word with a broader meaning than pocket book:

Other hypernyms of pocket book include paper-back book, paperback, paperback book, soft-cover, soft-cover book, softback.

pocket book (noun)	softback book (noun)
pocket-sized paperback book	a book with paper covers